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Spectrum sensing is the premise of cognitive radio, which determines the communication quality of the total system. For the cognitive nodes far away from the primary user, the probability of correct detecting drops clearly because of the great fading, no matter they spend more time sensing. Cooperation between relay nodes can enlarge the valid transmit distance and improve the quality of communication. This paper establishes the three-node cognitive radio system to do the optimization of spectrum sensing. The cognitive relay nodes forward the received signal after doing the sensing. In this condition, system's total bit error ratio decreases while the detect probability increases. There is an optimal sensing time when we take the system's total throughput as the target. With the cooperation of cognitive relay nodes, the optimal sensing time reduces.